1. Field of the Invention
The present invention relates to a platinum resistor temperature sensor.
2. Description of the Related Art
A conventional platinum temperature sensor as disclosed in U.S. Pat. No. 6,653,926 comprises a ceramic substrate; a platinum resistor formed of a meandering thin-film line provided on the ceramic substrate; a ceramic cover layer covering the platinum resistor; and a connecting layer for connecting the ceramic substrate so that the platinum resistor is encapsulated.
A process for manufacturing the aforementioned platinum temperature sensor includes applying a pressure to a ceramic green layer that becomes the connecting ceramic layer so that the platinum thin-film resistor is encapsulated between the ceramic substrate and the ceramic cover layer.
In the aforementioned conventional platinum temperature sensor, since the ceramic cover layer is connected to the ceramic substrate by the connecting ceramic layer that is made by applying a pressure and temperature to a green ceramic layer, as described in U.S. Pat. No. 6,653,926, an air-tight platinum temperature sensor capable of preventing the platinum thin-film resistor from an outer gas atmosphere entering inside is supposedly obtained.
However, when the above-described platinum temperature sensor is actually placed in an exhaust pipe of an internal combustion engine in order to detect the temperature of the exhaust gas exhausted therefrom, the platinum temperature sensor is exposed to a very high temperature exhaust gas possibly exceeding more than 1000° C. On this occasion, a small amount of a high-temperature oxidative gas of the exhaust gas passes through an interface between the ceramic cover layer and the connecting layer, through an interface between the ceramic substrate and the connecting layer or through the connecting layer per se having some porosity, and reaches the platinum thin-film resistor. As a result, a small amount of the platinum of the resistor evaporates, thereby increasing the resistance value of the platinum resistor in the long run, or in a worst case, losing the resistance across the terminals of the platinum temperature sensor.